I'm working on getting turbulators, and I have two questions for anyone who has them: 1) About how closely spaced are the twists? If you laid it on a flat surface, what would be the distance between the points where it touched? 2) How much clearance is there between the turbulator and the tube wall? Is it a pretty snug fit, or do they provide some clearance? I could get a whole series of them with different twists and clearances and gather exhaustive test data, but life is way too short.